Yesterday I found this really interesting video about why people work on open source projects. Actually, it's is applicable more generally on why people do volunteer work.
If someone makes enough money to not worry about money anymore, bonuses (as in more money) don't work anymore. In fact, a bonus will have a negative effect. If people don't have to worry about money anymore, self actualization and getting better at things you are good at and enjoy. Sounds like Maslovs pyramid, doesn't it?